Forst photo... how to improve?

I'm doing mostly photos of musical instruments and their parts for my website. Until recently I've been using my P&S Canon A570IS. I just bought a Canon Kiss X4

(aka 550D, aka T2i) and here is my first shot of an instrument with it. It's significantly better than anything I've ever got before. However I'd like to improve.

Some info: 18-55 IS kit lens, AV mode, f/3.5, 1/200, ISO 100. Clarinet was lyong on white sheet, in my porch (2nd floor apartment). Nautral but indirect sun light (was starting to lose the day light) plus pop-up flash. Camera hand held almost above the clarinet. Had no way to add lights to remove the shadow on the right side. I don't have Photoshop and used a free software I found online for slight editing (reducing size, cropping, etc.).

Main things I'm wondering about are improving the colours, background was really white but in photo not really and same for the colours of the clarinet, a hint of this strange colour (maybe WB wrong? I think I had it on Auto). Also would like to know if there's any way to improve sharpness of the photo? Maybe I'm choosing wrong A, S, ISO? Maybe I just don't hold it steady enough. I still don't really understand how to use DPP much. Is there a way to remove the dot reflections on the clarinet body (again, no Photoshop)?